1. Field
The present invention relates generally to identifying objects and in particular, the present invention relates to a method and apparatus for using electromagnetic signals to identify objects. Still more particularly, the present invention relates to a method and apparatus for identifying objects associated with non-linear radio frequency identification tags.
2. Background
Radio frequency identification (RFID) is an identification method that relies on storing and remotely retrieving data using devices called radio frequency identifier tags or transponders. A radio frequency identifier tag is an object that can be attached or incorporated into an object, such as a product, an animal, or person for the purpose of identification using radio waves. Currently, three different types of radio frequency identifier tags are present, passive tags, active tags, and semi-passive tags. A passive tag has no internal power supply. The incoming radio frequency signal provides the power needed for the circuit in the tag to power up and transmit a response.
Many passive tags generate a signal by back scattering the carrier signal from the reading. In other words, an antenna is designed to both collect power from the incoming signal and transmit the outbound back scatter signal. An active tag has an internal power supply that is used to power different circuits to generate or to control the outgoing backscattered signal. These types of tags are typically more reliable than passive tags.
Further, these types of tags are also able to transmit at higher power levels than passive tags because of the power supply located within the tag. As a result, active tags are more efficient in environments such as water, metal, and at longer distances. A semi-passive radio frequency identification tag uses an internal power source to monitor environmental conditions. This type of tag, however, requires radio frequency energy transferred to it from the reader interrogator to provide a response.
Currently used radio frequency identification systems have a limited range for identifying objects. Problems in range are encountered because of the frequency ranges used by currently available radio frequency identification systems. Lower frequency systems are typically in the 30 KHz to 500 KHz ranges. High frequency radio frequency identification systems use frequencies between 850 MHz to 950 MHz and 1.5 GHz to 2.4 GHz. These types of systems offer longer ranges that may reach around 100 meters. Within these ranges, the amount of power that may be transmitted by the radio frequency identification systems to identify tags are often limited by regulations to avoid interference with other devices that may operate in the same frequency bands.
The current ranges available are adequate in many applications, such as inventory control. The range, however, prevents the use of radio frequency identifiers in other applications that require longer ranges. For example, locating a lost individual or a downed pilot may require ranges of over a kilometer to make such a system effective.